While a refrigeration cycle device utilizing a Freon family refrigerant has been widely used as a multiple air conditioner for office buildings, a super critical refrigeration cycle utilizing a super critical fluid such as CO2 refrigerant is recently suggested to be installed in a multiple air conditioner for office buildings.
A super critical fluid is in a super critical state at the high pressure side, and the low pressure side is also at a higher pressure as compare to that of the Freon family refrigerant, so that the refrigeration system using the super critical fluid is a trans-critical cycle ranging over the critical point, providing a condition different from the conventional refrigeration cycle. Because of such the large difference between the high and low pressure, the input value of the air conditioning system needs to be large, and the super critical fluid generates a large temperature difference, different from the fluid of the vapor-liquid phase, so that, during the cooling operation when the outdoor air temperature is high, the temperature difference relative to the outdoor temperature is small, a sufficient heat exchange cannot be being performed, leading to an insufficient cooling, resulting in a COP inferior to that of the air conditioner utilizing the conventional Freon refrigerant.
Therefore, in order to suppress the high pressure at the compressor discharge portion and maintain the refrigerant ability of the super critical fluid, an expansion machine is installed and an intermediate cooler is utilized. An explanation will now be made as to a conventional example in which a second heat source side heat exchanger (second gas cooler) is used in the refrigeration cycle utilizing the second compressor driven by an expansion power recovered by an expansion machine. In the conventional example, an intermediate cooling system has been adopted, in which the second heat source side heat exchanger is disposed in a pipe between the first compressor and the second compressor, and the high pressure refrigerant compressed by the compressor is cooled by the second heat source side heat exchanger before it is compressed by the second compressor (see patent document 1, for example).
With such the construction, as compared to the compression stroke without using the intermediate cooling by the second heat source side heat exchanger, the intermediate two-stage compression needs less work for the compression, providing a higher COP for the same refrigeration capacity. Also, the COP during the heating operation is less improved than that during the cooling operation, so that the second heat source side heat exchanger is disposed in the outdoor unit and arranged to be operated only during the cooling operation in which a large improvement in efficiency can be obtained.
[Patent Document 1] Japanese Patent Laid-Open No. 2003-279179 (claim 5, FIG. 14, etc.)